The regulation of X-chromosome differentiation, whereby a single X-chromosome becomes inactive in each somatic cell of females will be characterized in the mouse. A microfluorometric assay for the X-linked gene product alpha-galactosidase will be used to study X-linked gene expression in males and female embryos before X-chromosome differentiation occurs. A genetic variant of phosphoglycerate kinase (PGK) has been found in feral mice and established in our mouse colony. We will use this variant to qualitatively determine whether X chromosomes are functional before the differentiation occurs. We have also established colonies of Mus species differeing from M. musculus for the X-linked genes hypoxanthine phosphoribosyl transferase (HPRT), PGK, and glucose 6-phosphate dehydrogenase. Interspecific hybrid embryos can be obtained by artificial insemination. These hybrids and the genetic variation for PGK in musculus will be used to characterize features of the differentiation process and its genetic regulation. Specifically, I will examine: 1) whether the process involves activation or inactivation of the X chromosome; 2) whether X-chromosome differentiation is intrinsic to the embryo or if it requires external signals; and 3) whether the inactivation is genetically regulated by autosomal genes or by loci on the X chromosome.